Cable deposits

The basic idea of analyzing the long term properties of in-containment cables by means of ongoing tests is not new. One of the advantages of cable deposits is that the cables age under real plant conditions but can, nevertheless, be checked and monitored.

4.1.1. Selection and installation of a deposit

It is important that the dose rate distribution in the NPP is known so that it can be used to select a position within the plant that is exposed to a higher dose rate than the real cable positions. It may even be possible to find a location where the ambient temperature is also similar to the design temperature (generally of the order of 50°C). Experience has shown that the loop line between the reactor pressure vessel and the steam generator is suitable for this purpose in pressurized water reactors and the reactor water cleanup system is suitable in boiling water reactors. In pressurized water reactors, the dose rate at this position is 1.3 to 1.5 times higher than the values prevailing at the most exposed cable positions.

If the cable samples are carefully arranged (e.g. if they are kept at a constant distance from the loop line as radiation source), the samples in the deposit are exposed to a homogeneous radiation field (Figure 2). The design of the deposit can easily be adapted to the local conditions in the power plant.

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Thermal insulation Reactor coolant une

Support strap for deposit rack

Cable samples

FIG. 2. Schematic cross-section of cable deposit layout around PWR coolant line.

The deposit should be equipped with a comprehensive selection of cables which are typical representatives of the types used in containment. The number of samples and their total length must also be analyzed in detail to ensure that enough material is available for the scheduled and unscheduled removal of samples to cover a period of up to 40 years. In this respect, the intervals at which samples are to be removed and the kinds of tests to be performed are of major importance. The following values can be used as standard: Samples of about 30 cm in length are quite satisfactory for measurements of elongation at break and, possibly, tensile strength in an irradiated condition. However, a minimum length of between 2 and 3 m is required for a DBE test with all electrical measurements.

The cable deposit is fitted with dosimeters to record and track the exact profile of the integrated radiation dose in the cable deposit. Care must also be taken to ensure that there is a free flow of oxygen into the deposit. Suitable contamination protection is desirable but must not restrict oxygen access.

4.1.2. Test procedures

First, it is essential to determine the elongation at break using a new (unaged) cable as reference. At intervals, samples are removed from the cable deposit and prepared for examinations and tests in a suitable test laboratory. The tests to be performed always consist of a sequence of the following steps:

(a) First, a measurement is made of the elongation at break of each sample.

(b) Then, the actual DBE resistance of the sample is determined.

(c) On completion of the DBE test, the elongation at break is measured again.

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In order to simplify testing and conserve the supply of sample material in the deposit, the actual DBE resistance is initially examined using a modified DBE test which is referred to as the

"steam test" in order to distinguish it more clearly. As long as the elongation at break determined in Step (a) is more than half of the elongation at break in the new condition (half-value dose still not reached), an approximately 30 cm long cable sample is exposed to the specified DBE temperature, pressure and moisture conditions. Electrical measurements are not performed during this test. The complete DBE test with all the normal electrical measurements is only performed when the half-value dose is exceeded. The results of Steps (a) to (c) must be recorded.

4.1.3. Determination of the sampling times and the deposit lead time

The determination of the sampling times can best be explained with the aid of the schematic diagram presented in Figure 3. In the upper section, the accelerated dose in the cable deposit (solid Une) is compared with the dose at the most exposed real cable position (dotted Une) as a function of time. The data for this graph are obtained from the dosimetry results for real positions and the cable deposit.

The results of the elongation at break test obtained from the cable deposit samples are plotted over the same time scale in the lower section of Figure 3 (soUd line). Assuming that there is no significant dose rate effect, the elongation values determined from the cable deposit can be shifted by the lead times (t,,,, t^.... etc.) to determine the variation in elongation values with time for cables in the rest of the plant. The specific test procedures used will be determined by the elongation values measured on the cable samples taken from the deposit, as discussed in Section 4.1.2. Since the acceleration factor for the cable deposit is small relative to installed cables, dose rate effects are likely to be insignificant.

Cable deposit

„ - Real position

^-ív7

40

lime (yrs)

t i

Time (yrs)

FIG. 3. Cable deposit methodology: determination of lead times for a cable deposit.

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For the types of cable currently in use, the first samples from the deposit could be removed five years after the start of plant operation since the type and qualification tests that have already been performed provide an acceptable confidence interval for this period at the least. The determination of further sampling times is dependent on the lead time of the cable deposit, as shown in the upper section of Figure 3, and changes in the test results. As long as the results show little change in elongation, resampling would seem to be sensible after another two to three years. If there is any deterioration in the test results, especially after the DBE test, it may be necessary to shorten the sampling interval accordingly.

4.1.4. Range of application

The above description assumes the most favourable case, namely, the installation of a deposit prior to or within 5 years of commissioning of the power plant. The conditions that are encountered in practice, however, may require modifications. For instance, a deposit can also be installed in a plant that is more than 5 years old. For this purpose, the cable samples to be placed in the deposit must be artificially aged in the laboratory with the lowest possible dose rate in order to attain the necessary lead time. Details of suitable laboratory ageing methods are given in Section 4.3.

The cable deposit methodology is not restricted to particular types of cables or materials.

The ongoing nature of a cable deposit programme means that the predictive information is relatively reliable, subject to changes in the environmental conditions in the plant.

A well-designed cable deposit should not affect the operation of the plant, although access to samples in practice would be restricted to normal maintenance outages. The type of cable deposit programme described in this section can be used to satisfy regulatory requirements, e.g.

various sections of the German KTA 3706 rule.

4.1.5. Cost of cable deposit methodology

The cost of initially setting up the cable deposit, maintaining unaged material and periodic testing (including DBE) will be relatively high compared with initial qualification testing. The increased reliability of lifetime prediction would, however, be particularly valuable, if the plant operators are aiming for life extension beyond 40 years.